The design of devices that operate at very low temperatures including, for example, the proposed Superconducting Super Collider (SSC), has brought about the need for new solutions to the problem of providing adequate structural support to massive components operating at such low or cryogenic temperatures. The SSC is an advanced proton-proton collider for use in high energy physics research that will consist of two 30 kilometer diameter accelerator rings housed in a common tunnel. The rings will accelerate protons to energies up to 20 TeV prior to their collision in particle detection facilities. In order to achieve these energies, the rings will incorporate superconducting magnets to bend the proton beam (dipole magnets) and to focus the beam (quadrupole magnets). The superconducting magnets operate at cryogenic temperature, i.e., about 4.5K, and are encased in cryostats or vessels for maintaining a vacuum and constant low temperature. Approximately eight thousand cryostats will be connected end to end to form the SSC accelerator rings. The cryostats and their components must therefore not only be mechanically reliable, but must also be manufacturable at low cost.
The cryostats play a crucial role in the overall performance of the SSC and other similar devices operating at very low temperatures. The cryostats must minimize heat leak from the outside environment to the superconducting magnets in order to maintain the required cryogenic operating temperature. In fact, the ultimate operating cost of the SSC may depend principally upon the ability of the cryostats to prevent heat leak to the magnets.
The major components of the SSC cryostat are the cryogenic piping, cold mass assembly (which includes the magnets), thermal shields, insulation, vacuum vessel, the interconnections between cryostats, and the system for supporting or suspending the cold mass assembly. The support system must maintain the position of the cold mass assembly during shipping, installation, repeated cooldowns and warmups of the magnets, and seismic excitations. In addition, the support system must be positionally stable over the expected 20 year operating life of the SSC and exhibit high impedance to heat conducted from the outside environment. The support system must also be inexpensive to manufacture and assemble, as well as easy to install and adjust. Very similar concerns apply as well to other devices operating at low temperatures, regardless of the particular construction or tasks performed by such devices.
In each cryostat, the cold mass assembly is supported within the vacuum vessel at several discrete points by support members. The number and location of these support members is determined by the need to distribute the static and dynamic loads of the cold mass assembly among several support members. In general, the number of support members is minimized in order to minimize heat leak at the support locations and to facilitate the fabrication and assembly of the cryostats.
In the final design of the SSC cryostats, the support members are multi-section support posts. These support posts are fixed at their base to the vacuum vessel, which is in turn anchored to the tunnel floor. The cold mass assembly is then mounted on the support posts. The invention, however, is not limited to this particular post-type design of the support members.
The cold mass assembly is usually anchored at one point along its length, typically at its mid-length, to one of the support members. This anchoring member serves to restrain the cold mass assembly from movement in the longitudinal as well as the lateral and vertical directions. The cold mass assembly must be slidably supported, however, by each of the other support members in the cryostat to allow for the contraction and expansion of the cold mass assembly in the longitudinal direction in response to the extreme temperature variations within the cryostat such as during cooldown and warmup of the superconducting magnets. Anchoring the cold mass assembly at these other support locations would impose intolerable bending loads on the posts during longitudinal contraction and expansion of the cold mass assembly.
As a result of the anchoring of the cold mass assembly at only one point along its length, force directed against the cold mass assembly in the longitudinal direction, such as during shipping, installation and seismic excitations, will be entirely concentrated upon the one anchoring support member. Such a concentration of longitudinal force may subject the anchoring member to excessive bending load and have a detrimental effect on the structural integrity of the anchoring member, and in extreme instances, may cause the anchoring member to fail.
Efforts in the past to counteract the bending load on the anchoring member have been directed to reinforcing the anchoring member. In the case where the anchoring member is a post, one know solution is to fit the anchoring post with a pair of angled reinforcing struts. This approach is illustrated in several SSC publications, including SSC Central Design Group, "Conceptual Design of the Superconducting Super Collider", SSC-SR-2020 (March 1986) at page 156, and R. C. Niemann et al., "Design, Construction And Test Of A Full Scale SSC Dipole Magnet Cryostat Thermal Model", 1986 Applied Superconductivity Conference (1986) at FIG. 4.
Such reinforcing struts extend generally diagonally from pivoted connections on the base or lower end of the anchoring post to pivoted connections on the cold mass assembly. Upon the imposition of force on the cold mass assembly in the longitudinal direction, the angled struts contribute resistive strength, and prevent the concentration of force at the upper end of the anchoring post and consequent bending load. However, the struts suffer from the inherent disadvantage of having to penetrate the thermal shields and multilayer insulation surrounding the cold mass assembly in order to connect the cold mass assembly to the base of the anchoring post. Consequently, the use of angled reinforcing struts increases the chances of radiative heat leak to the cold mass assembly and also increases the cost of manufacturing the cryostats because of the need to form special openings in the thermal shields and insulate the regions where the struts penetrate the shields.
The present invention is directed to overcoming these and other difficulties inherent in the prior art. In the present invention, a cryogenic support system is provided which includes tie bars connecting the anchoring post to the adjacent support posts which slidably support the cold mass assembly. The tie bars are mounted substantially parallel to the longitudinal axis of the cold mass assembly, and hence there is no penetration of the thermal shields and insulation surrounding the cold mass assembly, and heat leak is thereby avoided. Each tie bar comprises a rod formed of a material having a negative coefficient of thermal expansion, and end attachments which have a positive coefficient of thermal expansion.
As used herein, the term "negative coefficient of thermal expansion" indicates that the material expands or lengthens as it is cooled, and contracts or shortens as it is warmed. Conversely, the term "positive coefficient of thermal expansion" indicates that the material contracts or shortens as it is cooled, and expands or lengthens as it is warmed.
Very few materials possess a negative coefficient of thermal expansion. One such material is graphite in fiber form, which lengthens upon cooling from ambient temperature to cryogenic temperature. However, forming a structural element, such as a rod, tube or bar, out of graphite fibers requires the use of binder material, such as epoxy, as a substrate for the graphite fibers. These binder materials, including epoxy, shrink when cooled from ambient temperature to cryogenic temperature.
In order to produce a graphite fiber material in which the lengthening of the fibers exceeds the shrinkage of the binder material, one must align the fibers in the same direction. Such an arrangement results in what is termed a "uniaxial" composition. It is desirable to incorporate as high a volume content of the fibers as possible in the composition because when the volume content of graphite fibers is too low, the thermal behavior of the binder material will dominate, and the composition will shrink when cooled. On the other hand, if the fiber content is too high, the fibers will not adhere properly.
We have found that a graphite reinforced plastic (GRP) composition with a fiber content of about 50-55% by volume, when subjected to the pultrusion process, will yield a uniaxial structural element that is reasonably stiff and that increases in length when cooled from ambient temperature to cryogenic temperature. The pultrusion process involves drawing or extruding the material through a series of successively smaller rings or orifices to produce a structural element (rod, tube or bar) having fibers oriented in the same direction. The precise increase in the length of the bar when it is cooled to cryogenic temperature will depend primarily upon the volume content of the fibers and the nature of the binder material. We have found, however, that uniaxial GRP tubular elements are sufficiently stiff for use as tie bars in the present invention, and exhibit the desired increase in length, i.e., about 0.01% to about 0.05%, when cooled from ambient temperature (about 300K) to cryogenic temperature (about 4.5K).